High-performance carbon fibers are classified as either PAN-based carbon fibers obtained using polyacrylonitrile (PAN) as the starting material, or pitch-based carbon fibers obtained using a series of pitches as starting material. Carbon fibers are widely used for space and aviation, construction and civil engineering, as well as sports and leisure purposes, taking advantage of their extremely high strength and elastic modulus compared to ordinary synthetic polymers.
Carbon fibers have high thermal conductivity compared to ordinary synthetic polymers, and they are considered to have excellent heat spread properties. Carbon materials such as carbon fibers are also reported to exhibit high thermal conductivity by movement of phonons. Phonons are efficiently transmitted in materials with developed crystal lattices. Most currently marketed PAN-based carbon fibers do not have sufficiently developed crystal lattices, and because their thermal conductivity is lower than 200 W/(m·K), they are not necessarily satisfactory from the viewpoint of thermal management. Pitch-based carbon fibers, on the other hand, have a high degree of graphitization and well developed crystal lattices, and are therefore recognized as being more capable of achieving high thermal conductivity than PAN-based carbon fibers.
With the ever increasingly high densities of heat-generating electronic parts and the reduced sizes, thicknesses and weights of electronic devices including portable personal computers in recent years, demand continues to rise for lower heat resistance in the heat-spreading members used therein, and heat-spreading members with smaller thicknesses are strongly desired. Examples of heat-spreading members include heat-conductive sheets composed of cured products filled with heat-conductive inorganic powders, heat-conductive spacers composed of flexible cured products with heat-conductive inorganic powders that fill gelatinous substances, heat-conductive pastes with flow properties having heat-conductive inorganic powder filling liquid silicone, heat-conductive adhesives having heat-conductive inorganic powder filling curable substances, and phase change-type heat-spreading members that utilize resin phase transitions. The types of members among these that are more easily thinned are heat-conductive pastes, heat-conductive adhesives and phase change-type heat-spreading members, but for general-purpose articles it is preferred to use heat-conductive pastes or heat-conductive adhesives, based on their cost advantage and performance record. Heat-conductive adhesives have the advantage of facilitating attachment, and various forms thereof may be used.
For improved thermal conductivity of heat-conductive adhesives, a curable resin may be highly packed with a substance exhibiting excellent thermal conductivity, and a very low thickness formed. A low thickness can be formed by adjusting the viscosity of the adhesive and the size of the filler. As substances with excellent thermal conductivity, there are known metal oxides, metal nitrides, metal carbides and metal hydroxides such as aluminum oxide, boron nitride, aluminum nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, aluminum hydroxide and the like. However, metal material-based fillers have high specific gravity, and the weights of the heat-conductive adhesives are therefore increased.
Research is therefore being conducted on heat-conductive adhesives employing carbon materials with low specific gravity and high thermal conductivity, and especially carbon fibers.
Patent document 1 describes a heat-conductive filler composed of graphitized carbon fibers covered with a ferromagnetic material, the heat-conductive filler being characterized in that the graphitized carbon fibers are obtained using a mesophase pitch as the starting material for spinning, infusibility treatment and carbonization, and then pulverizing, followed by graphitization. Subsequent covering with the ferromagnetic material increases the orientation of the heat-conductive filler, the thermal conductivity being increased by orientation with a magnetic field. A strong magnetic field is therefore necessary for its use as a heat-conductive adhesive, and this has a particularly negative effect on electronic parts.    [Patent document 1] Japanese Unexamined Patent Publication No. 2002-146672